It is known that cold chamber pumps or hot chamber pumps can be used in the series production of precision casts by injection molding of non-ferrous metallic alloys such as of aluminum, zinc, magnesium, copper, etc. The use of cold chamber pumps, which are outside the crucible, in which the amount of alloy needed at every casting cycle is taken from the crucible and ladled into a non-heated cylinder, is subject to a series of serious drawbacks, such as the difficulty of dosing the amount of alloy, variations in the content of oxide which is taken from the molten alloy bath surface, irregular variations in the temperature of the alloy during the cycle, greater porosity of the pieces, etc.
Even if the use of hot chamber pumps, in which the injection pump is totally or partially immersed in the molten alloy, requires pumps of lower power and solves most of the above-mentioned problems yet it presents the great drawback that when said molten alloy at melting temperature is corrosive for the ferrous materials the members of the pumps are rapidly etched by it.
Therefore, it is apparent that the advantages offered by hot chamber pumps are such as to justify the continuous research of new corrosion-resistant materials, capable of assuring a sufficient life and reliability to the parts exposed to the contact with the corrosive alloys. Said materials are mainly obtained from alloys of various elements such as titanium, boron, silicon, carbon, chromium, aluminum itself and rarer elements such as yttrium, lanthanum, scandium, cesium, samarium, zirconium, etc. The processes used to obtain protective surfacings of alloys of said materials usually are sintering and slip casting, or the pieces themselves may be entirely made of anti-corrosion material. A number of examples of these corrosion-resistant alloys such as TiB.sub.2, ZrB.sub.2, TiC, Si.sub.3 N.sub.4, Y.sub.2 O.sub.3, La.sub.2 O.sub.3, etc. are described in the U.S. Pat. Nos. 3,319,702 and 3,586,095 (Union Carbide), in the Japanese patents JP81.000.389, JP56.023.358, JP56.023.359 and JP56.023.360 (Toshiba Mach.), and in the Swiss patent CH625.439 (Injecta).
The aim of the research of alloys more and more corrosion-resistant is that of extending the operating life of the pump, mainly as far as the most critical members such as the piston and the cylinder are concerned, which are not only subject to the corrosion by the molten alloy, but they also have to withstand the abrasion caused by the motion of the piston sealably sliding in the cylinder. The play which occurs between piston and cylinder owing both to the thermal expansion and the surface corrosion is extremely damaging for the correct working of the pump. In fact, the introduction of the molten alloy into the cylinder usually takes place through an opening in the side wall of the cylinder which is closed by the piston in its downward stroke with the consequent impossibility of using low rigidity piston rings which would be damaged by the passage on the side opening.
In the above-cited Injecta patent a pump is disclosed having a piston whose lower end is cut at 45.degree. so as to allow the inflow of the molten alloy into the cylinder without extracting completely the piston and without forming openings in the side wall of the cylinder, wherein, nonetheless, the piston must sealably slide, and therefore the problem of the coupling tolerances between piston and cylinder remains.
From the above it is apparent that in the prior art pumps special surfacings are needed for the critical coupling between piston and cylinder, in which account must be taken of the problems of thermal expansion, friction between the parts, corrosion of the contacting surfaces and possible oxide scales on said surfaces. The whole of these problems implies a shortening of the life of the above-mentioned critical members of the pump with consequent costs, both in terms of pieces replacement and machine stop times for the inspection and/or maintenance thereof.
The object of the present invention is to provide a pump for the hot chamber die casting of corrosive metallic alloys which is free from the above-described drawbacks thus allowing to considerably extend the nonstop working time of the machine.
This object is achieved by means of a pump having the features cited in claim 1.